1. Field of the Invention
The present invention relates generally to optical systems using a phosphor to convert light of one color to another color.
2. Description of Related Art
Light emitting diodes (LEDs) are a widely available, inexpensive, and efficient light source. However, a typical LED provides light of a single dominant color, with a very narrow spectral range.
It has previously been proposed to use a blue LED to excite a white phosphor, thus combining the simplicity of an LED as an electrically-powered light source with the better spectral output of a phosphor, or to use a blue LED to excite a yellow phosphor and combine the yellow light with unconverted blue light to produce a substantially white output beam. However, the LED prior art is also less than satisfactory regarding the geometry of phosphor utilization in LEDs, such as for LEDs that generate white light. Commonly-assigned U.S. Pat. No. 7,286,296 of Chaves et al. proposes novel configurations of manifolds for combining LED and phosphor light, but there is still room for further improvement.
The above-referenced concurrently filed application discloses dichroic filters for combining the blue light of an LED and the yellow light from a photostimulated patch of phosphor into a single white output beam. Because both the blue and yellow light as they emerge from the LED and the phosphor are uncollimated, no presently-available filter can efficiently combine their light until the range of incidence angles has been restricted. The most geometrically convenient beam-combination angle is 45°, but unless the light is polarized only a narrow angular range can be accommodated without loss of efficiency, typically 42-48°, whereas a realistic illumination system will exhibit a 30-60° range of angles into the filter. This ±15° range, however, is relatively narrow as collimation range goes, and cannot be expected to be smaller. While a 45° angle of beam combination is well known from RGB displays, it has not been possible to overcome the severe polarization effects that occur at this angle. In order to overcome this, several RGB display systems were developed where the band-pass operates at less than 45°.
A well known system that operates at a nominal 30° is the so-called Philips color TV camera. This system is described on pg. 1-79 of the book “Optical Coating Technology” by P. W. Baumeister, SPIE Press 2004. According to Baumeister (pg. 1-79) this “30°” design “has become an industry standard” and is based on U.S. Pat. No. 3,202,039, which describes the incidence angles as “smaller than 30°” without specifying the angle more precisely. Another approach is given in U.S. Pat. No. 3,905,684. In this prior art the band-pass filter operates at a nominal angle of 16° according to Baumeister (pg. 1-84) and therefore is an improvement over U.S. Pat. No. 3,202,039, as it is much easier to design a short or long pass filter that operates about this incidence angle. Baumeister states (pp. 1-84) that “The aforementioned Philips system has been used in a projection system . . . . In a modification [Cook 75], FIGS. 1-189 shows that an additional wedge prism is added to the entrance face, which permits the angle of incidence upon the blue dichroic (that follows) to be 16° for an axial ray. This is compared with the 30° angle for the system in §1.8.4.2.1 [the Philips system of U.S. Pat. No. 3,202,684]. It follows from the discussion . . . that this relatively small angle reduces the polarization splitting.”
The above-referenced concurrently-filed U.S. patent application of Falicoff discloses short-pass filters operating at a nominal incidence angle of 15°, and methods of designing such filters. The present application discloses manifold systems for combining un-polarized yellow and blue light based on a short-pass filter operating at a nominal incidence angle of 15°, for which the novel filters of the concurrently-filed application, while not essential, are especially suitable.